Magnetic Resonance imaging using the echo-volumar imaging approach (P. Mansfield, A. M. Howseman and R. J. Ordidge, Volumar Imaging (EVI) at 0.1 T, J. Phys. E. 22, 324-330 (1989) allows the simultaneous examination of several planes within a defined thick slice to be observed in the same time as would be necessary for the examination of a single plane in echo-planar imaging (P. Mansfield, A. M. Howseman and R. J. Ordidge, Volumar Imaging using NMR Spin Echoes, J. Phys. C. 10, L55 (1977) EPI. Following an initial thick slice selection process, the spin system is encoded along three principal axes, using suitably modulated x, y and z linear magnetic gradients. This type of echo-volumar imaging works well for homogenous specimens at high field and for less homogenous specimens at low field. The operating field strength comes into question since bulk susceptibility artefacts are induced, causing a degradation of the static field homogeneity. If this static field homogeneity is degraded too far, EVI as described above becomes impractical with the gradient strengths and switching rates currently available.
It is an object of the present invention to provide a means of overcoming the induced local field effects arising from susceptibility effects. The EVI modification uses 180.degree. RF pulses instead of gradient reversals. A similar modification has been described for use with Echo Planar Imaging EPI in British Patent Application 8918105.1.
The present invention therefore provides a method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process including subjecting the object to an initial selection process to select a thick slice within the object to produce an active volume of spin magnetisation characterized in that said method further includes subjecting the active volume so defined to a combination of 180.degree. RF pulses and suitably modulated x, y and z gradients to produce an echo volumar image.